Packed towers are used for mass transfer operations such as absorption, desorption, extraction, scrubbing and the like. The type of packing is chosen for its mechanical strength, resistance to corrosion, cost, capacity and efficiency. The function of the packing is to facilitate mass transfer between two fluid streams, usually moving countercurrent to each other. Efficiency and rate of mass transfer are enhanced by providing large surface area in the packing to facilitate contact of the fluids and by breaking the liquid into very fine droplets to enhance mass transfer to a gas phase.
Packing can be in the form of trays or packing bodies that are randomly packed into a column or tower. Originally, packing elements were ceramic or carbon rings, saddles, partition rings or drip point tiles. More modern packing bodies have a uniform distribution of open cellular units and provide higher efficiency and performance. They have very high wettable surface area and low resistance to fluid flow. They are effective in any orientation. The high efficiency packing bodies can be dump loaded into a column or tower and result in uniform distribution of the packing bodies without having blocked regions or void regions. These packing bodies permit streams to be processed at faster volumetric rates. Efficiency is increased and processing cost is reduced. The high efficiency packing bodies have complex dimensional shapes, usually with numerous struts and projections of different sizes and disposed at different angles and positions throughout the packing body.
However, the intricate structure of the uniform geometric shapes required for the high efficiency packing bodies requires that they be formed by casting, injection molding, stamping or extrusion, all expensive processes. Extrusion processes are limited since they generally are used to form shapes with axial symmetry. Also molding processes forbid the use of shapes such as undercuts and overlapping shapes since they cannot be released from ordinary molds. Multipart molds are prohibitively expensive. Thus, much of the internal volume is open space decreasing effective surface area. Baffle structure perpendicular to the longitudinal axis of the packing body is less than the optimum.
Metal packing bodies or elements are required for certain high temperature or chemically aggressive process streams. Most metal packing bodies are formed from metal blanks rolled into a tubular or spherical shape. Tabs or tongues may be cut and bent toward the interior to provide projections to increase surface area and enhance mixing and droplet formation. Again, there is substantial open area and efficiency is less than desired.
U.S. Pat. No. 4,724,593 describes an improved method for manufacturing high performance, symmetrical, open volumed packing bodies. The packing bodies have uniform geometrical configurations and are formed from a wide variety of materials into a wide variety of shapes and geometries. The process is simple and economical. A strip of sheet material has a pattern of repeating plates which are connected by intermediate ribbons of the sheet material. The plates may be perforated or contain projections. The plates are bent perpendicular to the longitudinal axis of the strip. The intermediate ribbons are then bent to bring the longitudinal axis of the bent plates into close proximity and in substantial parallel alignment.
The high performance packing bodies have performed well and have captured a significant share of the market. They have been manufactured in plastic or metal materials. These packings have low pressure drop, high mass transfer and packing efficiency. They have a high population of drip points per volume provided by a uniform distribution of surface elements. An open, non-obstructive structure provides low pressure drop while dispersing and distributing flow in both longitudinal and lateral directions.
While the void volume of the interior structure of the packing body is less than prior high efficiency packing bodies, the structure normal to the longitudinal axis is still difficult to provide and the manufacture requires several bending and rolling operations to form the sheet material into an element.
An improved packing body is disclosed in copending application, Ser. No. 08/147,806, filed Nov. 3, 1993, now U.S. Pat. No. 5,498,376, the disclosure of which is expressly incorporated herein by reference. The improved packing bodies are also formed from a strip of material. However, the perforated panels are not separated by ribbon connectors. A perforated strip of material is simply rolled into a spiral or into a concentric cylinder structure. The outer curved end of the strip is latched to the curved surface of the preceding revolution of the spiral. Baffle or tab elements disposed transverse to the surface of the strip efficiently disrupt the fluid stream. The tabs can be rod like elements raised from the surface. The improved packing bodies have a high degree of open space, from 30% to 98%. Surprisingly, the rolled packing bodies are found to provide better mass transfer and efficiency than prior packing body structures. However, it is difficult to automate rolling the strip into a spiral and latching the rolled element so that it does not unwind. Longer strips for large packings require a larger cavity to mold the strip.
Packing bodies have also been produced in a simplified manner from elongated apertured strips as disclosed in Ser. No. 08/229,698 filed Apr. 19, 1994, now U.S. Pat. No. 5,458,817, the disclosure of which is expressly incorporated herein by reference. The strips are formed into segments and the segments on each side of a medial segment are folded toward the top surface of the medial segment and segments on the other side are folded toward the bottom surface of the medial segment. The segments may be provided with single or double fold lines to facilitate folding the strip material without bending or stressing the strip material.
Though packings made from a folded strip are easier to manufacture than the packings formed from a rolled strip, the packings are limited in complexity and size.